Graphene Actuators: Quantum-Mechanical and Electrostatic Double-Layer Effects

The electrochemical actuation of covalent carbon materials, such as graphene, immersed in liquid electrolytes has shown immense promise for a myriad of applications. To realize this potential, an intimate understanding of the physics behind the actuation is essential. With the use of ab initio density functional calculations, it is shown that the strain induced in monolayer graphene by the formation of an electrostatic double-layer (DL) is the dominant actuation mechanism. The DL-induced strain (∼1%) is found to exceed the quantum-mechanical strain (∼0.2%) due to charge injection only, for charges and electric potentials of greater than −0.08 e/C-atom and 1 V, respectively. Various methods of calculating the graphene atomic charges based on first principle charge densities are compared and contrasted. The electrochemical charge-strain and potential-strain relationships for monolayer graphene are shown to be parabolic in nature. This study proves that the origin of the high electrochemical strains in covalent carbon materials is the electrostatic DL potential, and demonstrates the true viability of using monolayer graphene for nanoelectromechanical systems (NEMS) actuators

High-Performance Graphene Oxide Electromechanical Actuators

Having demonstrated unparalleled actuation stresses and strains, covalently bonded carbon-based nanomaterials are emerging as the actuators of the future. To exploit their full potential, further investigations into the optimum configurations of these new materials are essential. Using first-principle density functional calculations, we examine so-called clamped and unzipped graphene oxide (GO) as potential electromechanical actuator materials. Very high strains are predicted for hole injection into GO, with reversible and irreversible values of up to 6.3% and 28.2%, respectively. The huge 28% irreversible strain is shown to be the result of a change in the atomic structure of GO from a metastable clamped to more stable unzipped configuration. Significantly, this strain generation mechanism makes it possible to hold a constant strain of 23.8% upon removal of the input power, making this material ideal for long-term, low-power switching applications. A unique contraction of unzipped GO upon electron injection is also observed. It is shown that the origin of this unique behavior is the modulation of the structural rippling effect, which is a characteristic feature of GO. With reversible strains and stresses in excess of 5% and 100 GPa, respectively, GO is poised to be an extremely useful material for micro/nanoelectromechanical system actuators